Commercially practical fiber-MOPA systems are now capable of delivering pulsed output with pulses having a duration as short as one picosecond or even a few hundred femtoseconds (fs). A common configuration of such a fiber-MOPA includes a mode-locked fiber-laser delivering a train of seed-pulses having the desired pulse-duration at a repetition rate in the tens of megahertz (MHz); optionally, one or more stages of fiber-amplification for initially amplifying the pulses; a device for stretching the duration of the pulses; a modulator or “pulse-picker” for selecting amplified pulses from the train for further amplification; one or more stages of fiber-amplification for power amplification; and a pulse compressing device for returning the duration of the power-amplified pulses to about the duration of the seed-pulses. Fiber-amplifiers offer high-gain and efficiency, and, for the most part, are easily packaged.
In fiber-MOPAs delivering such pulses with an average power on the order of 10 or more Watts (W), final fiber-amplifier stages typically include a rod-fiber or so-called photonic-crystal fiber (PCF). These fibers are expensive and can contribute as much as 25% to the total cost of a fiber MOPA. Further such fibers are subject to degradation through photo-darkening and require sophisticated packaging and cooling techniques to avoid thermal issues and pump-induced stresses that lead to modal instabilities and fiber damage.
It has been proposed to use one or more bulk solid-state amplification stages, either in a multiple-pass configuration or in form of a regenerative amplification scheme, in place of rod fibers or PCFs. These bulk amplifiers which include end-pumped rods, face-pumped disks, or side-pumped slabs, however, are relatively low in gain, and inefficient compared with fiber-amplifiers, and, accordingly, would require more powerful and more complex pump arrangements and beam handling arrangements, more powerful seed lasers, besides simply adding volume and cost to the MOPA.
End-pumped Yb-doped rod amplifiers, for instance, require very high brightness pump light and are output power limited to about 10 W due to strong thermal lensing and high temperature in the gain material. Yb-doped disk amplifiers operate at low temperature, due to the large cooling surface, and can be pumped at power levels of up to several kW. However, they exhibit very low gain factors of between about 1.1 and 1.3 per seed-beam interaction, requiring up to 150 interactions (bounces) in a regenerative amplifier to amplify a milliwatt level seed beam to several tens of Watts.
Side and end-pumped Yb-doped slab amplifiers have been proposed as an alternative. However, due to Yb being a quasi 3-level system, a high pump intensity (on the order of about 10 kW/cm2) is required to generate gain over a 10 mm long, 10 mm wide and 200 um thick pump region. This means that pump powers in excess of 150 W are required just to make the slab transparent, making this geometry only efficient and cost-effective for high-power multi-pass amplifiers with several 100 W of output power. In addition, the requirement of very high brightness of the pump light in one direction leads to very complex and expensive beam trains for the pump light.
There is a need for an alternate replacement for rod and PCF fibers in a fiber MOPA. The replacement preferably retains the high-gain, efficiency, and ease of packaging offered by fiber-amplification.